This invention relates to a tilt device for a marine propulsion unit and more particularly to an improved hydraulically operated tilt and trim unit wherein the trim cylinder can be moved to a retracted position when the outboard drive is tilted up and wherein the incorporation of this system will not adversely affect the normal operation of the device.
As is well known, many marine outboard drives employ hydraulic or power operated systems for effecting trim movement of the outboard drive between a fully trimmed down condition and a fully trimmed up condition and a tilt arrangement for moving the outboard drive from its fully trimmed up condition to a tilted up out of the water condition. One very popular form of system of this type employes a trim cylinder that is affixed to the transom of the watercraft and which has abutting engagement with the outboard drive for effecting the trim movement of the outboard drive. In addition, there is provided a tilt cylinder that has a connection to the watercraft and to the outboard drive and which is operative to tilt the outboard drive up to its fully tilted up out of the water condition from beyond the fully trimmed up condition.
Normally these systems are operated by a hydraulic circuit in which both the tilt and trim cylinders are operative throughout the trim range and wherein the tilt cylinder continues to operate the outboard drive when the trim cylinder is at the end of its stroke so as to achieve the tilting up operation. This type of arrangement means that the piston rod of the tilt cylinder is fully exposed when the outboard drive is tilted up beyond the fully trimmed up condition. There are a number of disadvantages to such an arrangement. Primarily, this means that the trim cylinder rod may be fully exposed for long periods of time even when the outboard drive and associated watercraft is not being operated. As a result, there can be corrosion or encrustation occur or other difficulties which can attack the piston rod of the trim cylinder and render it either inoperative or inefficient.
Therefore, there has been proposed a hydraulic system which, in addition to the aforenoted operation, permits the trim cylinder to be retracted once the tilt cylinder has the outboard drive in its tilted up condition. Normally, this is achieved through the addition of a further circuit between the actuating pump and the retraction side of the trim cylinder which includes a check valve that opens at a lower pressure than the pressure required to operate the tilt cylinder in a down direction. As a result, operation of the pump in a down mode will effect retraction of the trim cylinder without effecting retraction of the tilt cylinder.
Although the described modified hydraulic system for achieving trim cylinder retraction avoids the problems with the more conventional systems as aforenoted, it itself gives rise to certain difficulties. For example, the addition of this additional circuitry and its normally closed condition causes the system to be rendered sensitive to the intrusion or inclusion of air in the system during initial filling. As a result, air in the system can cause difficulties in operation. In addition, the provision of the valving as described can cause leakage of fluid out of the system and disadvantages in operation.
It is, therefore, a principal object of this invention to provide an improved tilt and trim cylinder assembly that will not present any of these disadvantages.
It is a further object of this invention to provide an improved tilt and trim cylinder arrangement and actuating system therefor where normal tilt and trim operation can be accomplished, the trim cylinder can be retracted when the outboard drive is tilted up and wherein air in the system will be self-purging and, furthermore, the likelihood of intrusion of air into the system during operation is reduced.